The Boston Symphony Orchestra’s performances of Mahler’s Third Symphony this past March were notable for myriad reasons. They demonstrated, as well as anything last season, that the orchestra has maintained a high standard of ensemble despite, at that point, having been without a music director for two years. The concerts also fueled speculation that the conductor, Daniele Gatti, had an inside track on the music director job. (He didn’t; it went to Andris Nelsons.)

But the performances were important for another reason, one having to do with a solo that happens around the middle of the symphony’s third movement. Mahler wrote the solo for a posthorn, a brass instrument that originated in the 18th century and was used to signal the approach of mail coaches. The solo is played offstage by a trumpeter, usually on a trumpet or variant thereof, such as a flugelhorn or cornet. Many listeners at the BSO performances found an unusual degree of warmth, depth, and mystery in the solo. “The sound,” Jeremy Eichler wrote in a Globe review, drifted “in as from a distant world.”

There was a good reason for this impression. Thomas Rolfs, the BSO’s principal trumpet, was for the first time using an actual posthorn, which he purchased with those performances in mind. The instrument, rarely made today, added that extra quantum of rapture to the sound. Now, as the BSO prepares to play the Third on opening weekend at Tanglewood, this time under Rafael Frühbeck de Burgos, Rolfs is preparing to tackle the solo again on the instrument Mahler originally had in mind.

Speaking by phone from Tanglewood recently, Rolfs said he bought the posthorn last summer. It was made by a German brass instrument maker named Ricco Kuhn, and Rolfs tried it out at the Jamaica Plain office of Kuhn’s American representative, Ken Pope.

“I just fell in love with the sound of it,” Rolfs said. “So I bought it. And then I tried to figure out how to play it.”

That turned out to be easier said than done. Where a trumpet is cylindrically shaped, the posthorn is conical, and it looks something like a miniature French horn. When he first got the instrument, Rolfs joked with the BSO’s horn players that he thought playing the French horn was easy. “And then as I got closer to the [Mahler] concert, I decided to let them know that French horn isn’t quite that easy.”

In fact, it was very much like learning a new instrument. “When you blow through a brass instrument, you shape the airstream,” Rolfs explained. “Over time, you get the airstream that makes the sound that you have in your head. And because of the conical nature of the instrument, and the odd mouthpiece, it’s just a different airstream, and it took me a while to get used to that.”

He worked with the posthorn through the fall. When Gatti came to Boston in January to conduct the Verdi Requiem, Rolfs told him, “ ‘I really want to try this horn, but I’m having trouble playing it in tune, and I’m missing notes.’ And so [Gatti] said, ‘I want you to be comfortable, I want you to be able to sing on it. You make the decision and don’t worry about it.’ And that was a great answer.”

At the first Mahler rehearsal, Rolfs wasn’t happy with his performance of the solo, and he told Gatti that he was thinking of switching to a trumpet. But this time, Gatti responded, “No, no! I love the sound! You have to figure it out now!” Rolfs recalled.

In the end, the solos in the Mahler performances were almost universally praised. The magic came from the way the instrument’s soft-bored sound seemed to surround Symphony Hall’s listeners in their seats.

“People in the audience said they had a hard time even identifying where the sound was coming from,” Rolfs said. “If you were playing a trumpet, it’d be a little more pointed, a little easier to locate. This was more ethereal. Imagine a big hall and the sound just floating through the hall and you’re not sure where it’s coming from.”

With Saturday’s performance looming, Rolfs said he feels somewhat more confident about the solo than on the instrument’s maiden voyage. “And as I say that,” he added, “I want to find some wood to knock on. Trumpet players and horn players — we don’t like to talk about these things. We tiptoe around them.” There’s a still a sense for the trumpeter that he’s playing something that isn’t his primary instrument.

“The minute I start playing it like a trumpet, I start missing notes and it starts going out of tune. That’s my big fear in playing the solo — not because my ego is wrapped up in this, but because I don’t want to ruin the moment for all the listeners. I want the moment to be really special, and if I start missing notes the moment is gone.”

The posthorn was a significant investment for Rolfs. He wasn’t specific about the cost beyond saying that it was “multiple thousands of dollars” — not surprising for a handmade instrument of this type.

But, he added, “how many more times in my career am I going to get to play the posthorn solo? At this stage of my career, I want to be able to retire and say, I’m really glad I did that. And it was an incredible experience to play this instrument. So it’s totally worth it.”

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